The two images below correspond to two processed seismic cross-sections. My question is this: why at/near 0 Hz is there "lots" of seismic energy still present? Most if not all of the lower end of the bandwidth stops at 10 - 15 Hz.
Typically seismic data should oscillate around 0 amplitude in the time domain, i.e. there should be just as much positive amplitude as there is negative (otherwise the ground would have had a net displacement in some direction). This is the energy at 0 Hz in the amp spectrum. It is like a bulk shift (DC shift) on the data.
So... unless the ground was permanently displaced by your seismic source, this is probably just an artifact. It could be as simple as: you used too few data to calculate your amplitude spectrum (e.g. small window, etc). Or it could be a super low frequency artifact coming from data processing.